Metal oxide semiconductor (MOS) technology is used to form a number of different types of devices which include semiconductor memory devices dependent on hot electron programming. When programming one type of these devices, hot electrons are injected from the drain through the tunnel dielectric and into the floating gate. As such, hot electrons are present during programming and do not give rise to any adverse effects. On the other hand, when a high hot electron concentration occurs in a MOS transistor, several problems can arise including hot electron induced device degradation. Hot electrons may have sufficient energy to damage the substrate-gate dielectric interface near the drain edge that may cause to adverse changes in the transistor characteristics.
Certain types of intermetal dielectrics have been found to cause problems with hot electrons at the substrate-gate dielectric interface near the drain. In the prior art, annealing intermetal dielectric layers for densification and driving out moisture is not performed. An intermetal dielectric (IMD) film typically absorbs ambient moisture and when the device is passivated, the moisture is trapped. Subsequent heat cycles may drive the moisture into the gate oxide region. In the prior art, the hot electron susceptibility of transistors using wet gate oxides is known to be inferior to those of transistors using dry chlorinated gate oxides. Therefore, water absorbed into the IMD film could migrate to the gate dielectric thereby making a device more susceptible to hot electron induced device degradation.
A number of prior art attempts have not yielded adequate results, one of which includes placing a moisture barrier in the form of gate sidewall spacers comprised of a material such as silicon nitride near the gate of the device. Silicon nitride causes device instability because hot electrons are trapped in the silicon nitride and near the gate.
Another prior art attempt minimizes the exposure of the IMD film to air between the steps of IMD film deposition and passivation deposition. This processing sequence has serious drawbacks in a manufacturing environment because it is typically difficult to guarantee that the equipment used for subsequent steps would be available immediately following the IMD deposition. As such, a sufficiently short queue time between IMD deposition and passivation deposition which is typically a few processing steps later in the process flow has proven to be impractical.
A high density IMD film absorbs ambient moisture at a rate greater than desired, although the ambient moisture absorbance rate is less than a low density IMD film. Because of the ambient moisture absorbance rate, a short queue time between IMD film deposition and passivation deposition may be required even for a high density IMD film. Even if a queue time can be tolerated, enough ambient moisture may be absorbed and cause the hot electron induced device degradation previously described.